<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .parent{
            border: 2px solid #333;
            width: 100px;
            height: 100px;
            display: flex;
            justify-content: center;
            align-items: center;
        }
        .test{
            width: 40px;
            height: 40px;
            background-color: pink;
        }
        .parent1{
            border: 2px solid #333;
            width: 100px;
            height: 100px;
           position: relative;
        }
        .test1{
            width: 40px;
            height: 40px;
            background-color: pink;
            position: absolute;
            left: 0;
            right: 0;
            top: 0;
            bottom: 0;
            margin: auto;
        }
        .parent2{
            border: 2px solid #333;
            width: 100px;
            height: 100px;
        }
        .test2{
            width: 40px;
            height: 40px;
            background-color: pink;
            margin-top: 30px;
            margin-left: 30px;
        }
    </style>
</head>
<body>
    <div class="parent">
        <div class="test"></div>
    </div>
    <br/>
    <br/>
    <div class="parent1">
        <div class="test1"></div>
    </div>
    <br>
    <br>
    <div class="parent2">
        <div class="test2"></div>
    </div>
</body>
</html>